This disclosure sets forth a design for integrally fabricated gated triode pixel elements and the associated control circuitry for flat panel or virtual reality displays. In addition, methods for manufacture of the integrated devices are disclosed.
Prior art electroluminescent (EL) devices used in flat panel displays are diodes in which applied alternating current (AC) or direct current (DC) or pulse potentials affect luminescence. The diode or two terminal embodiment of flat display panel pixels presents significant operational and manufacturing limitations. One limitation is in the form of barrier contacts at both the injector and the collector terminals of the diode. These barrier contacts increase significantly the potential required for luminescence, and decrease the operational lifetime of the device because of cumulative terminal EL material interface stress. The stress of the high field of the non-ohmic contacts to the EL material affects the interface therebetween, degrading operation and causing failure. Another limitation is the increased complexity of address and intensity modulation necessary for use as pixel elements in information display.
Still another limitation is that the address and intensity modulation circuitry must be separately manufactured and assembled to prior art diode pixel display devices thereby increasing the cost of the display product. Another limitation is the power requirements for the control circuitry which are orders of magnitude greater than the control circuitry for the presently disclosed device. Still another limitation of the diode pixel element is that light is emitted through the diode""s transparent contact and not laterally as will be expanded upon in the following paragraph. This results in a significant percentage of light emission that is not utilized thereby, further increasing the power required in the prior art to obtain the desired level of luminescence.
In prior art DC diode devices, one contact to the EL material of the diode is made by transparent indium-tin oxide, and the other by a metal which is typically Al. Both of those contacts are Schottky barrier, tunneling contacts. A reverse bias applied to the EL material produces a field across the depletion region. A sufficient field causes avalanche of energetic carriers which are typically electrons. The electrons impact and excite centers, or color centers of the EL material, creating electron-hole pairs, and/or excitation of the color dopant atoms. Relaxation of the excitation within the EL material causes photon, colored light, emission. Only the photons exiting the EL material parallel to the field produce the viewed light. The greater brightness produced laterally, perpendicular to the field, is essentially lost and does not markedly contribute to the brightness of the viewed light of the prior art.
The current invention comprises triode pixel devices and complementary triode logic devices for control of pixel devices. Both the pixel devices and the associated control circuitry are fabricated and interconnected in the same continuous manufacturing process to economically produce full color flat panel display products. Both pixel and logic devices are operated in a gate controlled avalanche mode.
Pixel elements are formed of inorganic or organic EL material ohmically contacted by low work function metal. The depletion region necessary for controlling EL intensity or for preventing EL avalanche is affected by potentials to a gate element injected into the EL material. The shape of the gate element multiplies the field produced by applied gate potential. Luminescence is directly viewed through the glass substrate, without an indium tin oxide layer and its 10% light transmission loss, from the brighter, lateral EL emission, not available in the prior diode pixel art. Each pixel element can be surrounded by an optically absorbing black oxide, the equivalent of a TV tube""s black mask, increasing pixel contrast and definition. The complementary logic devices are formed from separate depositions of n-type and p-type silicon with their respective gates connected in common. The operating potentials required are those of integrated circuits and are therefore low. Power consumption is reduced and the devices present no electromagnetic hazard to users. The ohmic contacts to EL material and the gate terminal of the present disclosure overcome operating lifetime and failure problems of the prior art DC operated EL devices. Those failure mechanisms, which are overcome by the present disclosure, are well described by J. M. Blackmore, et al., Journal of Applied Physics 61, No. 2, p.714-733.
To achieve the aforementioned objects, uses and advantages, a deposited mixture of metal and oxide particles interfaces with semiconductors and/or oxides. The mixture is sputter deposited in a 10% hydrogen 90% argon atmosphere. The random mixture of particles of 50 xc3x85 maximum diameter is graded such that 28% to 32% of the receiving surface area is metal particles. At barrier contact to the semiconductor depletion region, the oxide particles isolate a multiplicity of metal particles of the mixture such that fields about the metal particles are enhanced over that obtained between the essentially planar surfaces of the prior art. That enhanced field increases tunneling current density at a given field potential. The multiplicity of tunnel current sources, as compared to the prior art, provides a more uniform and additional increase in tunnel current density. In contact to highly doped semiconductor, the mixture makes better micro-ohm-cm2 contact to the semiconductor, enhancing device operation and speed over the prior art. The mixture prevents migration of metals into semiconductors. The simple and economic process disclosed replaces difficult and less reliable silicide formation in the semiconductor of prior art contact processes. The ratio of 30% metal particles is optimum. A range of 28-32% can be routinely achieved. The range can be extended to about 25-40% with a loss of benefit. In general terms, the 30% figure is best for optimum tunneling.
The particle mixture improves the ohmic contact to interfacing oxide layers. The ohmic contact is used to eliminate one of the two serial barriers that applied fields otherwise overcome to tunnel charge stored in floating gates. Elimination of one barrier lowers potentials and increases device life.
The metal and oxide materials of the random mixture are chosen such that the work function of the oxide is sufficiently greater than the work function of the metal whose other characteristics combine to enable ohmic contact with oxides.
The preferred oxide is typically defined by the oxide used in the manufacturing process of the device. Silica is the preferred oxide for interfacing with silica or silicon. Other oxides, which like silica make ohmic contact with the preferred metal, are alumina and beryllia.
The preferred metal Cr3Si is an A15 compound, congruently melts at 1770xc2x0 C., has a coefficient of thermal expansion of 10.5xc3x9710xe2x88x926/xc2x0C. typical of a silicide, and does not oxidize at temperatures below 1050xc2x0 C. The heat of formation of Cr3Si of xe2x88x9232.4 kcal/mole correlates to a barrier of 0.55 ev to either N-doped silicon or P-doped silicon. The advantage of equal barriers to oppositely doped silicon is not ordinarily achieved in prior art IC processes. However, the conductivity of Cr3Si is about 1/15th that of aluminum, so that more conductive materials are used in contact with Cr3Si for interconnections to other IC elements. Alone or in a mixture with oxides, the very high free surface energy of Cr3Si provides strong adherence to many common materials used in IC manufacture, and an effective barrier to the migration of metals (e.g., aluminum) into semiconductor. The prior art teaches formation of silicides into semiconductors to bar such migrations which cause failure of devices, but the siliciding process itself is a source of failure.